In sample analysis instrumentation, and especially in separation systems such as gas or liquid chromatography and capillary electrophoresis systems, smaller dimensions will generally result in improved performance characteristics and at the same time result in reduced production and analysis costs. In this regard, miniaturized planar devices provide more effective system design and result in lower overhead due to decreased instrumentation sizing. Additionally, miniaturized planar devices enable increased speed of analysis, decreased sample and solvent consumption and the possibility of increased detection efficiency.
Several approaches towards miniaturization have developed in the art. The conventional approach provides etched planar devices on glass, silicon, metal, or ceramic substrates of moderately small size. For example, planar devices may be etched in a wafer that receives a superimposed cover plate. In some approaches, certain fluid handling functions have not been successfully integrated into the planar device and accordingly must be effected by the attachment of conventional devices, such as fused silica capillary tubing, to the planar device.
More recent approaches have used micromachining of silicon substrates and laser ablation of organic nonmetallic substrates to provide structures of much smaller size (i.e., microstructures) on the substrate. For example, there has been a trend towards providing planar systems having capillary separation microstructures. See, for example: Karasek, U.S. Pat. No. 3,538,744; Terry et al., U.S. Pat. No. 4,474,889; Goedert, U.S. Pat. No. 4,935,040; Sethi et al., U.S. Pat. No. 4,891,120; Shindo et al, U.S. Pat. No. 4,905,497; Miura et al., U.S. Pat. No. 5,132,012. Silicon provides a useful substrate in this regard since it exhibits high strength and hardness characteristics and can be micromachined to provide structures having dimensions in the order of a few micrometers.
A drawback in the silicon micromachining approach to miniaturization is the chemical activity and chemical instability of certain materials used for substrates, such as silicon, quartz or glass, which are commonly used in systems for both capillary electrophoresis (CE) and chromatographic analysis systems. Accordingly, Kaltenbach et al., in commonly-assigned U.S. Pat. No. 5,500,071, and Swedberg et al., in commonly-assigned U.S. Pat. No. 5,571,410 disclose a miniaturized total analysis system comprising a miniaturized planar column device for use in a liquid phase analysis system. The miniaturized column device is provided in a substantially planar substrate, wherein the substrate is comprised of a material selected to avoid the inherent chemical activity and pH instability encountered with silicon and prior silicon dioxide-based device substrates. More specifically, a miniaturized planar column device is provided by ablating component microstructures in a polymer substrate using laser radiation. The miniaturized column device is described as being formed by providing two substantially planar halves having microstructures thereon, which, when the two halves are folded upon each other, define a sample processing compartment featuring enhanced symmetry and axial alignment.
However, although the foregoing techniques are useful in the fabrication of miniaturized planar devices for effecting fluid handling functions in sample analysis systems, there are significant disadvantages to the prior art approaches. One significant problem remains in providing exact alignment of complementary pairs of microstructures that are respectively provided in a planar substrate and its cover plate, or in a pair of planar substrates, when such microstructures are intended to be superimposed so as to form one or more channels capable of performing a fluid handling function in a unitary assembly. For some applications, prior art planar technology has not produced a sufficient degree of alignment between the superimposed microstructures.
Another problem arises in the attempt to effect hermetic sealing of the superimposed surfaces. This step is generally carried out using adhesives which may not fully isolate the channels thus resulting in cross-channel leakage. Conventional approaches can be prone to failure, leakage, or to degradation induced by adverse conditions, such as high temperature environments, or by the destructive nature of certain gases or liquids that may be present in the channels.
Still another problem arises because silicon substrates, and most ablatable materials such as polyimides, do not offer a sufficient combination of thermal and mechanical characteristics such that the substrate is usable in certain applications. For instance, silicon materials are not ductile and cannot be folded, shaped, etc.; ablatable materials exhibit a low coefficient of thermal conductivity and are not susceptible to rapid and uniform heating or cooling, nor do they offer sufficient strength or ductility such that an ablatable substrate may be configured as a connecting member, housing, or support for other components in a sample analysis system. Ablatable materials are expressly selected for their propensity to ablate upon the application of heat, and thus are not considered to be as robust and impervious to adverse (e.g., high-temperature) environments in comparison to metals and metal alloys.
Another problem arises in a prior art microminiature valve that is constructed of "hard" (i.e., non-resilient) materials, such as silicon, in the valve seat and/or the movable member that contacts the valve seat. Such valves can fail when one or more fluid-borne particles of sufficient size becomes lodged between the valve seat and the movable member, and effectively precludes the valve from closing. To accommodate such an occurrence, some designs incorporate "soft" (i.e., resilient) materials; however, resilient materials cannot withstand the high-temperature environment and harsh chemicals typically encountered by a valve situated in a modern chromatographic instrument.
Accordingly, there is a need for a novel microminiature valve, and an integrated assembly incorporating such a valve, having improved pneumatic characteristics and thermal characteristics, and which is less susceptible to failure due to a high-temperature environment, the presence of harsh chemicals, or to particles which may block proper operation of the valve.